1. Field of the Invention
The present invention relates to a negative dispersion mirror and a solid-state laser apparatus including the negative dispersion mirror. Particularly, the present invention relates to a soliton-type mode-locked solid-state laser apparatus, which has a small size, and which can perform short-pulse operation.
2. Description of the Related Art
Conventionally, solid-state laser apparatuses using semiconductor lasers (LD), as excitation light sources (pump light sources), and solid-state laser media (laser crystal, ceramics or glass) doped with rare-earth ions or transition-metal ions, as laser media, have actively been developed. Especially, application of short-pulse laser apparatuses, which generate so-called short pulse light (short pulsed light) in the picosecond to femtosecond regime, to various fields such as medicine, biology, machine industries and measurement has been sought and proposed. Further, some of the short-pulse laser apparatuses have been practically used after their efficiency was confirmed.
This kind of laser apparatus generates short pulses by an operation called as mode-locking. In brief, the mode-locking is a phenomenon in which when a laser oscillates, the phases of all of a multiplicity of longitudinal modes (axial modes) are locked in the frequency regime (relative phase difference=0). Therefore, multimode interference between the longitudinal modes occurs and extremely short pulses are generated in the time regime.
Particularly, in soliton-type mode-locking, which is an example of CW (continuous wave oscillation) mode-locking, negative group-velocity dispersion in a laser cavity (resonator) and self-phase modulation mainly in a laser medium are combined and pulses in the femtosecond regime can be generated.
Basically, the solid-state laser apparatus that can realize the soliton-type mode-locking includes a solid-state laser medium, a saturable absorber mirror and a negative group-velocity dispersion device, which are provided in a cavity (resonator). In the following description, the “negative group-velocity dispersion” may simply be referred to as “negative dispersion”.
Conventionally, one of a prism pair (a pair of prisms), a diffraction grating pair (a pair of diffraction gratings), a negative dispersion mirror and the like or a combination thereof has been used as the negative group-velocity dispersion device.
As the negative dispersion mirror, there are a chirp-type mirror (chirp mirror) and a GTI (Gires-Tournois interferometer) type mirror. The chirp-type mirror performs negative dispersion compensation (in other words, compensates positive dispersion by performing negative dispersion compensation) by utilizing a difference in the penetration depth between light having a long wavelength and light having a short wavelength. The GTI-type mirror performs negative dispersion compensation by utilizing interference of light between a total reflection mirror and a partial reflection mirror.
A typical example of the chirp-type mirror is a mirror in which high refractive index layers, which have a relatively high refractive index, and low refractive index layers, which have a relatively low refractive index, are alternately deposited one on another. In the chirp-type mirror, these layers are deposited in such a manner that the optical thickness of each of the high refractive index layers and the low refractive index layers linearly changes in the direction of deposition of the layers (for example, please refer to R. Szipöcs et al., “Chirped multilayer coatings for broadband dispersion control in femtosecond lasers”, Optics Letters, Vol. 19, No. 3, pp. 201-203, 1994.
Meanwhile, the characteristic of the GTI-type mirror is that a resonation structure is provided within the dielectric multilayer coating (please refer to J. Xuhl and J. Heppner, “Compression of Femtosecond Optical Pulses with Dielectric Multilayer Interferometers”, IEEE Transaction on Quantum Electronics, Vol. QE-22, No. 1, pp. 182-185, 1986 for example). Further, a double GTI structure mirror that has two cavity layers within the multilayer coating has been proposed (please refer to International Patent Publication No. WO00/25154). Further, a mirror that has no cavity layer, but the multilayer coating of which is structured in such a manner that it has a resonance structure has been proposed (please refer to International Patent Publication No. WO00/11501). In the mirror that has no cavity layer, the multilayer coating is structured in such a manner that the optical thickness of each layer forming the multilayer coating changes according to a certain pattern or rule so that the resonance structure is obtained.
Further, in Japanese Unexamined Patent Publication No. 2(1990)-023302, a dielectric multilayer coating that performs not only second-order dispersion compensation but third-or-higher-order dispersion compensation has been proposed. The third-or-higher-order dispersion compensation is performed by depositing at least two stacks of dielectric multilayer coatings one on another. In each of the stacks, at least two kinds of layers that have different refractive indices from each other are alternately deposited one on another. Further, each of the stacks is formed in such a manner that the central wavelength thereof differs from each other. Further, in Japanese Unexamined Patent Publication No. 2000-138407, a multilayer coating mirror that has a reflectance of greater than or equal to 95% in the visible light band has been proposed. In the multilayer coating mirror, the refractive index of the outermost layer is set lower than that of a layer that is immediately under the outermost layer. The multilayer coating mirror is formed in such a manner that negative group velocity dispersion occurs.
Further, in Japanese Unexamined Patent Publication No. 11(1999)-168252, a technique of providing a chirp mirror coating on a laser medium, a saturable absorber or an output mirror has been proposed to reduce the size of the mode-locked solid-state laser apparatus.
The inventors of the present invention have discovered that a negative dispersion mirror that can compensate at a greater negative dispersion value than a conventional mirror and that has a sufficient reflectance as an output mirror is necessary to reduce the size of a soliton-type mode-locked solid-state laser apparatus.
However, the negative dispersion values of conventional negative dispersion mirrors are approximately in the range of minus tens to minus hundreds fs2. Therefore, it has been necessary to provide a plurality of mirrors in a resonator, if necessary.
Further, in Japanese Unexamined Patent Publication No. 11(1999)-168252, a technique of providing a negative dispersion function in the output mirror has been proposed. However, Japanese Unexamined Patent Publication No. 11(1999)-168252 fails to specifically disclose the optical transmittance, negative dispersion value and the like of the mirror in the case of using the mirror as the output mirror. Further, a coating that constitutes the mirror is not specifically described. Further, in Japanese Unexamined Patent Publication No. 2000-138407, the feature that the frequency chirp can be compensated by providing a dielectric multilayer coating in the output mirror is disclosed. However, the negative dispersion value of a multilayer coating that is disclosed as a concrete example is extremely low. Therefore, if a single device (element) is used, sufficient negative dispersion is not obtained. Further, since the reflectance is greater than or equal to 99.9%, which is very close to 100%, substantially no output light is obtained Hence, the function as the output mirror is not sufficient.